In recent years, touch panels (coordinate sensors) have been in wide use as a device for entering data into electronic instruments having multi-functionality such as portable phones, PDAs (Personal Digital Assistant), MP3 players, and car navigation systems equipped with mid- to small-size displays. Touch panels are also increasingly used for larger display devices.
Conventionally, most touch panels used in such devices having touch panels are resistive or capacitive touch panels. These conventional touch panels, however, have some problems. For example, because they need a special panel for detecting locations, the entire device becomes thick. Also, the touch panel provided in the display region of the display device negatively affects the visibility.
To solve these problems, in place of above-mentioned resistive or capacitive touch panels, display devices with built-in touch panel, i.e., so-called two-dimensional sensor array, have been developed, where light-receiving elements (light sensor elements) such as photodiodes and photo transistors are included in the display area of the display device.
However, one problem with the configuration mentioned above is a reduced aperture ratio, which is caused by a large number of light receiving elements arranged two-dimensionally in the display area. Also, the light signal read-out circuit is very complex.
To solve the problems, display devices with a built-in coordinate sensor have been developed. In those display devices, bus lines (scan signal wiring and data signal wiring) for display elements (drive element) such as TFTs (Thin Film Transistors) provided in the display region also serve as bus lines (scan signal wiring and data read-out wiring) for light-receiving elements. Here, display and sensing are conducted in a time-division manner, providing a display device with a built-in coordinate sensor that can suppress the aperture ratio reduction.
However, in the above-mentioned configuration, because display and sensing are conducted in a time-division manner, the operation speed is restricted.
For this reason, display devices with built-in optical scanning touch panel without the above-mentioned two-dimensional sensor array has been developed. Such display devices do not have the above-mentioned problem of reduced aperture ratio or restricted operation speed.
In Patent Document 1, for example, an optical scanning touch panel is disclosed. This optical scanning touch panel, as shown in FIG. 11, includes light-emitting/receiving units 101a and 101b, which are disposed outside of respective end portions of one short side (the side at right in the figure) of a rectangular display screen 100 to be touched by a pointer such as a finger or a pen (blocking item) S. The light-emitting/receiving units 101a and 101b respectively include light-emitting elements 111a and 111b, light-receiving elements 113a and 113b, polygon minors 116a and 116b, and the like, which constitute an optical system. Over the outer surfaces of the three sides of the display screen 100 except the side at right, a retroreflection sheet 102 is disposed.
In the light-emitting/receiving unit 101b of the above-mentioned optical scanning touch panel, the scanning of the light projected from the light-emitting/receiving unit 101b starts at the position at which the light directly enters the light-receiving element 113b, continues counterclockwise in FIG. 11 to the position at which the light projected from the light-emitting/receiving unit 101b is blocked by the light-shielding member 170 provided to prevent the light from entering the light-emitting/receiving unit 101a, and further proceeds to position (Ps) at which the light is reflected by an end portion of the retroreflection sheet 102. Then, the light is reflected by retroreflection sheet 102 before position (P1) at which the light encounters one end of pointer S, but after position (P1) and before position (P2) at which the other end of pointer S is reached, the light is blocked by pointer S. After position (P2) and until the later scanning position (Pe) is reached, the light is reflected by the retroreflection sheet 102.
According to the configuration described above, the light reflected by the retroreflection sheet 102 is detected, and the range in which the detection level of the returned light is smaller than a predetermined threshold is detected as a shielded range where the light is blocked by a finger or a pen.
In the disclosure, it is stated that the threshold setting may be changed frequently according to the noise in the light reception system, operation environment, and the like to improve the accuracy in the calculation of the location and size of the pointer S.
According to the configuration disclosed in Patent Document 1, the above-mentioned calculation accuracy is improved by using the following scheme.
When a light reception level below the predetermined threshold is not detected, that is, when pointer S is determined not present, for a prescribed length of time, the OFF signal is output from MPU to the light-emitting element driver circuit to stop the light-emitting operation of the light-emitting elements 111a and 111b. During this time, MPU adds a predetermined value (margin voltage) to the light reception output from the light-receiving elements 113a and 113b measured while the light-emitting elements 111a and 111b are OFF, and this value is set as the threshold so that any influence of the ambient environmental light can be eliminated.
The margin voltage is determined based on the fluctuation in the amount of the light received due to the noise in the light reception system, digitalization error occurred during the A/D conversion, and the accumulated time-series light reception data, and the like.
In this configuration, when pointer S is determined not present, the scanning light is turned off for a prescribed period of time to conduct the threshold setting process, and therefore, it is stated that, the threshold can be set without interfering the derivation of the location and size of the pointer S.